1. Field of the Invention
The present invention relates to a surface-mounting coil component applied, for example, to coils for heightening and lowering voltage of DC/DC source of portable electronic devices.
2. Description of the Related Art
A current corresponding coil (such as choke coil) for application to DC/DC power source of the portable electronic devices such as portable telephones or digital still cameras has been in particular demanded to have a surface-mounting coil component of low height in an external dimension while securing a desired inductor characteristic.
The portable electronic device is usually carried around and subjected to severe changing of circumstances in temperatures, and therefore a surface-mounting coil component mounted on a board housed inside of the portable electronic device is imposed heat cycle tests of 10 cycles at −25° C. to +85° C., or most severely, 10 cycles at −40° C. to +85° C.
As representative structures of the surface-mounting coil component used to the existing portable electronic machinery, a sleeve core is covered on the outer circumference of the drum-type ferrite core to which the winding is wound around the winding core portion connecting the upper flange and the lower flange, the sleeve core is fixed by an adhesive with terminal electrodes of a metal frame, and both ends of the winding are fixedly bound and soldered on the terminal electrode (not shown).
Further, as other existing surface-mounting coil components, there are the surface-mounting coil components of a structure solely composed of the drum-type ferrite core wherein the winding is wound around the winding core and both ends of the winding are conductively connected to plane external electrodes directly attached to the core, or of a structure of filling an resin coating material to cover around the winding between both flanges of the drum-type ferrite core.
As the structure of the conventional surface-mounting coil component, the under mentioned [Patent Literature 1] describes the structure of a coil part using the drum-type ferrite core as shown in FIG. 6, a perspective view from the bottom side.
That is, the coil part 10 has the structure comprising the drum-type ferrite core 8 that is composed of the upper flange 4 and the lower flange 2 extended to set on both upper and lower ends of the winding core 1 with a vertical winding axis, two pairs of external electrodes 3a, 3b, 3c, 3d, being furnished in the lower flange 2 of the drum-type ferrite core 8, and the windings 5, 6, being wound around the winding core 1 of the drum-type ferrite core 8 and having both ends 5a, 5b, and 6a, 6b respectively connected to the external electrodes 3a, 3b, 3c, 3d by soldering or thermal press-attaching.
[Patent Literature 1] Laid Open No. 115023/1995
Upon progressing reduction of height in surface-mounting coil components using the conventional drum-type ferrite core, in a type of using the drum-type ferrite core and a sleeve core, the sleeve core is disposed adjoining the circumferences of both flanges of the drum-type ferrite core. Since this type appears similar to the structure of a closed magnetic circuit, although it is advantageous in the coil characteristics (in particular, L: inductance), it is disadvantageous in cost and reduction in height since more number of parts are required.
On the other hand, in the conventional surface-mounting coil component 10 shown in FIG. 6, for realizing reduction in height and concurrently providing the current corresponding coil having a desired inductor characteristic, it is necessary to cover the outer circumference of the winding wound around the winding core between the flanges with the resin coating material with magnetic powder of 60 to 90 wt % in order to secure a necessary capacity of the winding and form an effective magnetic path around the winding.
For producing the surface-mounting coil component of the outside dimension of 1.2 mm or lower using the simplex drum-type ferrite core, the prior art took a technique of bringing a linear expansion coefficient of the drum-type ferrite core and a linear expansion coefficient of resin coating material with magnetic powder to the closer value.
However, in the surface-mounting coil component by the above-mentioned conventional technique, with respect to the flange of the drum-type ferrite core which is 0.35 mm or less in thickness, and has a value of 1.9 or more of a ratio L2/L1 an outer diameter L2 of the upper flange to a diameter L1 of the winding core of the drum-type ferrite core (the flange in the present pertinent surface-mounting coil component, corresponding to such a flange having the maximum overhang size exceeding 1.0 mm in the diameter direction from the outer circumference of the winding core of the upper flange of the drum-type ferrite core), strength of the flange of the drum-type ferrite core could not counter work the stress arising due to the difference between the linear expansion coefficient of the drum-type ferrite core and the linear expansion coefficient of the resin coating material with magnetic powder in the heat cycle tests (−25° C. to +85° C., 10 cycles, or −40° C. to +85° C., 10 cycles) which is generally required for the parts of portable electronic devices, and the flanges could not avoid inconvenience of cracks occurring.
Further, in the producing process, due to hardening and shrinking of the resin coating material with magnetic powder when filling and hardening this resin on the outer circumference of the winding wound around the winding core between the flanges of the drum-type ferrite core, the flanges also had inconvenience of cracks occurring.